A positron emission mammography (PEM) instrument is being developed to be used as a clinical and research tool for the diagnosis and planning of treatment for breast cancer. Dedicated instrumentation for PEM was implemented in a standard x-ray mammography unit by positioning two gamma cameras, one above and one below a compressed breast. The radiotracer fluorodeoxyglucose (FDG) is administered to the patient. FDG is known to concentrate in breast tumors, thus making them detectable in positron emission tomagraphy (PET) systems. FDG emits gamma rays, which are detected by the gamma cameras. Each camera has a last-dynode output connected to a coincident circuit for event trigger. The data from each camera is analyzed and placed in memory. Images of the distribution are obtained in real time and displayed. The system parameters include: (1) real-time image capability, so that a 1-cm hot spot can be seen within 30 seconds; (2) resolution of a 1-cm-diameter hot spot in a phantom, so that a hot spot-to-background ratio will be 8:1 or greater; and (3) a high singles rate in the range of 100 KHz with a minimum separation of pulses, to avoid overlap, in the 1.0- to 1.5-sec range. The instrument development includes the design and fabrication of the detectors, signal-processing and data-acquisition electronics, and real-time programming for acquisition, display, and storage of events.